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PENSION AND OTHER BENEFIT PLANS - Fair Value of Plan Assets by Assets Category (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Total      
Defined Benefit Plan      
Fair value of plan assets [1] $ 266,529 $ 243,704  
Total | Level 1      
Defined Benefit Plan      
Fair value of plan assets [1] 233,070 221,953  
Total | Level 2      
Defined Benefit Plan      
Fair value of plan assets [1] 33,459 21,751  
Total | Level 3      
Defined Benefit Plan      
Fair value of plan assets [1] 0 0  
Money Market Accounts / Cash Equivalents      
Defined Benefit Plan      
Fair value of plan assets 13,441 18,625  
Money Market Accounts / Cash Equivalents | Level 1      
Defined Benefit Plan      
Fair value of plan assets 13,441 18,625  
Money Market Accounts / Cash Equivalents | Level 2      
Defined Benefit Plan      
Fair value of plan assets 0 0  
Money Market Accounts / Cash Equivalents | Level 3      
Defined Benefit Plan      
Fair value of plan assets 0 0  
Equity Securities      
Defined Benefit Plan      
Fair value of plan assets 147,763 128,539  
Equity Securities | Level 1      
Defined Benefit Plan      
Fair value of plan assets 147,763 128,539  
Equity Securities | Level 2      
Defined Benefit Plan      
Fair value of plan assets 0 0  
Equity Securities | Level 3      
Defined Benefit Plan      
Fair value of plan assets 0 0  
U.S. Large-cap      
Defined Benefit Plan      
Fair value of plan assets 66,709 52,682  
U.S. Large-cap | Level 1      
Defined Benefit Plan      
Fair value of plan assets 66,709 52,682  
U.S. Large-cap | Level 2      
Defined Benefit Plan      
Fair value of plan assets 0 0  
U.S. Large-cap | Level 3      
Defined Benefit Plan      
Fair value of plan assets 0 0  
U.S mid/small cap      
Defined Benefit Plan      
Fair value of plan assets 23,127 16,432  
U.S mid/small cap | Level 1      
Defined Benefit Plan      
Fair value of plan assets 23,127 16,432  
U.S mid/small cap | Level 2      
Defined Benefit Plan      
Fair value of plan assets 0 0  
U.S mid/small cap | Level 3      
Defined Benefit Plan      
Fair value of plan assets 0 0  
CBU common stock      
Defined Benefit Plan      
Fair value of plan assets 5,691 6,427  
CBU common stock | Level 1      
Defined Benefit Plan      
Fair value of plan assets 5,691 6,427  
CBU common stock | Level 2      
Defined Benefit Plan      
Fair value of plan assets 0 0  
CBU common stock | Level 3      
Defined Benefit Plan      
Fair value of plan assets 0 0  
International      
Defined Benefit Plan      
Fair value of plan assets 50,270 51,725  
International | Level 1      
Defined Benefit Plan      
Fair value of plan assets 50,270 51,725  
International | Level 2      
Defined Benefit Plan      
Fair value of plan assets 0 0  
International | Level 3      
Defined Benefit Plan      
Fair value of plan assets 0 0  
other      
Defined Benefit Plan      
Fair value of plan assets 1,966 1,273  
other | Level 1      
Defined Benefit Plan      
Fair value of plan assets 1,966 1,273  
other | Level 2      
Defined Benefit Plan      
Fair value of plan assets 0 0  
other | Level 3      
Defined Benefit Plan      
Fair value of plan assets 0 0  
Fixed Income Securities      
Defined Benefit Plan      
Fair value of plan assets 92,566 82,198  
Fixed Income Securities | Level 1      
Defined Benefit Plan      
Fair value of plan assets 59,107 60,447  
Fixed Income Securities | Level 2      
Defined Benefit Plan      
Fair value of plan assets 33,459 21,751  
Fixed Income Securities | Level 3      
Defined Benefit Plan      
Fair value of plan assets 0 0  
U.S. Treasury and Agency Securities [Member]      
Defined Benefit Plan      
Fair value of plan assets 74,598 56,562  
U.S. Treasury and Agency Securities [Member] | Level 1      
Defined Benefit Plan      
Fair value of plan assets 59,107 49,665  
U.S. Treasury and Agency Securities [Member] | Level 2      
Defined Benefit Plan      
Fair value of plan assets 15,491 6,897  
U.S. Treasury and Agency Securities [Member] | Level 3      
Defined Benefit Plan      
Fair value of plan assets 0 0  
Investment Grade Bonds [Member]      
Defined Benefit Plan      
Fair value of plan assets 17,968 18,502  
Investment Grade Bonds [Member] | Level 1      
Defined Benefit Plan      
Fair value of plan assets 0 3,648  
Investment Grade Bonds [Member] | Level 2      
Defined Benefit Plan      
Fair value of plan assets 17,968 14,854  
Investment Grade Bonds [Member] | Level 3      
Defined Benefit Plan      
Fair value of plan assets 0 0  
High Yield [Member]      
Defined Benefit Plan      
Fair value of plan assets [2]   7,134  
High Yield [Member] | Level 1      
Defined Benefit Plan      
Fair value of plan assets [2]   7,134  
High Yield [Member] | Level 2      
Defined Benefit Plan      
Fair value of plan assets [2]   0  
High Yield [Member] | Level 3      
Defined Benefit Plan      
Fair value of plan assets [2]   0  
Other Investments [Member]      
Defined Benefit Plan      
Fair value of plan assets [3] 12,759 14,342  
Other Investments [Member] | Level 1      
Defined Benefit Plan      
Fair value of plan assets [3] 12,759 14,342  
Other Investments [Member] | Level 2      
Defined Benefit Plan      
Fair value of plan assets [3] 0 0  
Other Investments [Member] | Level 3      
Defined Benefit Plan      
Fair value of plan assets [3] 0 0  
Dividends and Interest Receivable [Member]      
Defined Benefit Plan      
Fair value of plan assets 900 600  
Pension Benefits      
Defined Benefit Plan      
Fair value of plan assets 267,462 244,332 $ 290,687
Pension Plans      
Contribution made to defined benefit pension plan by employer $ 4,300 $ 100  
[1] Excludes dividends and interest receivable totaling $0.9 million and $0.6 million at December 31, 2023 and 2022, respectively.
[2] This category is exchange-traded funds representing a diversified index of high yield corporate bonds.
[3] This category is comprised of exchange-traded funds and mutual funds holding non-traditional investment classes including private equity funds and alternative exchange funds.